WO 2011/101019 A1 describes an impeller-type mechanical coolant pump with an outlet valve arrangement between the outlet volute and the pump outlet. When the valve is closed, the coolant delivery is more or less completely stopped because a high closing force is generated by the coolant pressure and allows high valve sealing forces. High opening forces are, however, needed to open the valve flap against the pressure of the coolant pushing the valve flap in the closed position.